The key to building muscle, getting lean or boosting your endurance.

Whether your fitness goal is building muscle, burning fat or boosting endurance, the good news is that you can achieve it by eating the same quality proteins, carbohydrates and healthy fats from whole, unprocessed sources that you should be already. (You are, right?) Just in different amounts.

“It’s a case of changing a couple of variables and trying to keep everything else consistent so you can monitor the results,” says Mr Matt Gardner (mattgardnernutrition.com), former performance nutritionist for Premiership rugby club Worcester Warriors, who now runs his own consultancy for active individuals.

First off, you’ll need to calculate your daily calorie requirements via an app or website (Mr Gardner suggests the Total Daily Energy Expenditure Calculator), adjust your intake depending on your goal and track your progress. You’ll also need to become familiar with your kitchen and bathroom scales. Admittedly, this may not be as appealing as the latest fad diet. “But it really works,” adds Mr Gardner.

Oh, and unless your endurance event is the Beer Mile – four laps of a running track with a 355ml brewski necked before each – alcohol is not particularly conducive to any of these.

Below, according to expert opinion, are three diets that will help you achieve your fitness goal.

To get bigger, you need to take in more than you burn off. Start with a calorie surplus of 100-500 per week and monitor your progress. You also need to do weight training three to five times a week, and sleep seven to nine hours a night. Assuming you do lift, Mr Gardner recommends eating 2g of protein per kilo of bodyweight per day, spread across, say, three main meals and two snacks, each of which contains a minimum of 20g. The key is to ensure a constant supply of muscle-building amino acids (the building blocks of protein) to your system. However, don’t skimp on the carbs, which not only fuel your lifting but also aid protein absorption. Or fats, which are high in calories and thus the hard-gainer’s friend. “Nuts, seeds, eggs, avocados and tinned fish are also portable,” points out Mr Gardner, who has prescribed high-fat diets to bulk up academy rugby players. Generally speaking, carbs can be anything from 20-45 per cent of your intake, and fat 25-45 per cent, with protein making up 30-40 per cent. Aside from protein powder, he suggests supplementing with essential or branched-chain aminos and 5g a day of creatine monohydrate, which improves power and recovery. So you can lift and lift some more.

To get smaller, you need to burn off more than you take in. “The master regulator for fat loss is creating an energy deficit,” confirms Mr Gardner. If you haven’t already, ditch the “biochemical battering rams” of highly refined carbs and sugars, which will also eliminate a ton of calories. You can cut your intake by an aggressive 20 per cent, or start slowly with five per cent, see how your body responds and build up. Carbs and fats are the obvious macros to salami-slice, but Mr Gardner advocates capping protein at 1.2-1.5g per kilo of body weight: more and your body may use it for fuel rather than retaining muscle. Time your larger meals after training: wake up and perform a fasted “low-and-slow” cardio session of 30-40 minutes at a pace where you can hold a conversation, first sipping a coffee or green tea to encourage your body to tap into its fat stores. (Caffeine is the active ingredient in most fat-burning supplements, which should be avoided.) Not a Frappuccino, mind.

This comes down to timing in a different sense. “If you are training for longer than 90 minutes for a marathon or something similar, you should look at consuming carbs beforehand or during your workout to give your body the energy to move through that session effectively,” says Mr Gardner. That could be three-quarters of a cup of oats, grains or potatoes prior, or 30-60g of carbs per hour during. Mr Gardner, who has gotten into ultra-running, is a fan of “priming your gut” by having a nibble 20-30 minutes in, whether food, liquid or gel, to kick-start the digestive process and prevent stomach upset later. Weigh yourself before and after to monitor dehydration and replenish accordingly (1kg equals 1 litre of water). You should aim to replace 120 per cent of what you’ve lost over time – if you neck it all in one go then it’ll be peed out again. If you’re a salty sweater, consider an additional pinch of Maldon or an electrolyte drink with sodium; another key electrolyte is magnesium, found in avocados and dark leafy greens. Drinking 250ml of beetroot juice daily for two weeks before the event you’re training towards has been found to enhance performance, as can cutting caffeine down or out then reintroducing it. Otherwise, don’t drink or eat anything new or unusual on the day – or else.